There is a long term trend in seismic reflection surveying for oil and gas exploration and production to utilize sensing elements, commonly known as geophones, at decreasing spatial sample intervals. There is a continuing need for economical ability to measure seismic wavefields at finer spatial sampling. The need for economical and efficient acquisition of seismic data is particularly significant for surveys acquired on the bottom of a body of water such as the ocean. There is a need for finer spatial sampling to improve the imaging of the geologic subsurface. Also there are particular modes of seismic noise and interfering signals on the water bottom that are better ameliorated with finer spatial sampling of the seismic wavefields.
Ocean Bottom Seismic (OBS) surveys are a well-established technology. This technology encompasses Ocean Bottom Cable (OBC), and Ocean Bottom Node surveys (OBN). It is common in OBS surveys to record and analyze so-called Dual-Sensor data consisting of a scalar pressure measurement, and an effectively co-located vector component measurement of vertical particle motion, such as particle velocity or acceleration. Dual-Sensor data will be understood here to mean a pressure (P) measurement, and a co-located nominally vertical (i.e., Z-Cartesian axis) particle motion measurement. Said Dual-Sensor data is sometimes referred to as P-Z data. There is a common practice in Ocean Bottom Seismic to utilize Dual-Sensor data to separate up-going and down-going seismic waves to remove multiples and for other purposes in data processing and analysis.
There is also recognition that measurements of vertical pressure gradient may be used in conjunction with measurements of pressure to separate up-going and down-going seismic waves.
There are Ocean Bottom Nodes available for recording seismic data on the water bottom. These are commercially offered by, for example, Fairfield, Oyo-Geospace, Seabed Geosolutions, and Fairfield Nodal.
There are Ocean Bottom Cables available for recording seismic data on the water bottom. These are commercially offered by, for example, Sercel and Ion Geophysical.
Marine seismic surveys are commonly recorded by utilizing pressure sensitive hydrophones. Hydrophones are widely used in towed streamer surveys. There is also an emerging commercial technology of utilizing pressure gradient measurements in towed streamers to enhance the spatial sampling of the pressure seismic wavefields. See, for example, U.S. Patent Application No. 2009/0040871 to Morley entitled “Wide Tow Enabled by Multicomponent Marine Seismic Cable”.
There is a well established technology for measurement of the linear particle motion of seismic wavefields in the earth. Many commercial sensors exist to measure particle velocity or particle acceleration along one, or up to three, linear axes, utilizing various physical concepts to accomplish the measurements. It is most common to utilize measurements of the vertical particle motion. On the water bottom linear particle motion sensors are commonly deployed, typically along with pressure sensing hydrophones, in Ocean Bottom Cables or in Ocean Bottom Nodes.
There is a common practice in multi-component seismic to numerically rotate the components to different spatial orientations. For example, components recorded by sensors in arbitrary but known orientations may be rotated to a coordinate system of North, East, and Vertical; or they may be rotated to a coordinate system with one axis perpendicular to a local reference plane that is a smooth approximation of the water bottom.
There is an evolving commercial technology for measurement of the rotational particle motion of seismic wavefields in the earth. See, for example, U.S. Pat. No. 7,516,660 to Kozlov entitled “Convective Accelerometer” and U.S. Pat. No. 8,024,971 to Kozlov entitled “Convective Accelerometer”. This includes sensors such as those commercially offered by, for example, MetTech (model Metr-3), Eentec (models R-1 and R-2), and Applied Technology Associates.
The utility of rotational seismic measurements is appreciated in earthquake and regional crustal seismology, as discussed, for example, in Lee, W., et. al., eds., 2009, Rotational Seismology and Engineering Applications, Bull. Seismological Society of America, vol. 99, no. 2B, supplement, May, 2009. Seismic rotational motion is commonly understood to be the vector curl of the infinitesimal displacement field. The existing rotational sensors are understood to measure the components of this vector curl.
There is an evolving commercial technology for measurement of the linear components of the spatial gradient of pressure. This includes so-called vector hydrophones such as those commercially offered by, for example, Applied Physical Sciences and BenthoWave. Other pressure gradient measurement technology is disclosed in U.S. Pat. No. 7,295,494 to Meier entitled “Diamagnetic Current Response Transducer for Sensing Pressure Gradient in a Fluid Medium”. Spatial gradients may also be measured as differences between properly calibrated hydrophones that are deployed in an appropriate geometric array. Applications are discussed in disclosures such as International Patent Application No. WO 2012/015520 to Meier entitled “Seismic Acquisition Method for Mode Separation”.
The significant effect of the water bottom on stress fields, strain fields, and seismic wave fields is widely understood. These concepts are described, for example, in Aki, K., and Richards, P., 2002, Quantitative Seismology, University Science Books, p. 128 ff., pp. 184-185. The shear modulus of water is commonly understood to be effectively zero for seismic wave propagation. The shear stress components commonly referred to as σxz and σyz, involving the nominal vertical direction z, normal to the water bottom for a nominally horizontal water bottom, have zero value at the water bottom. For significant variations of the water bottom from a horizontal orientation, measured data may be numerically rotated to an orientation with components perpendicular and parallel to a reference plane that appropriately approximates the water bottom.
In the technical field of sampled data analysis, there is a well established technology for enhanced sampling rate by utilizing the sampling of the wavefield in conjunction with the spatial gradient of the wavefield being sampled. This technology is extensible to multiple spatial dimensions. This technology may be implemented by various algorithms that may include ability to handle irregular sampling, and the ability to optimally handle the effects of noise in the data. Persons having ordinary skill in the art will appreciate that there are many algorithms that may be employed to reconstruct spatial sampling. One widely known reference for the fundamental concept of Ordinate and Slope Sampling is Bracewell, R., 2000, The Fourier Transform and its Applications, McGraw-Hill, pp. 230-232. Other known algorithms may be utilized to deal with effects such as irregular sampling, or the effects of noise in the data.
There is technology to utilize rotational sensors in conjunction with vertical linear particle motion sensors to enhance the spatial sampling of that single vertical linear component of motion for a seismic wavefield. For example, see U.S. Patent Publication No. 2012/0113748 to Brune entitled “Method to Improve Spatial Sampling of Vertical Motion of Seismic Wavefields on the Water Bottom by Utilizing Horizontal Rotational Motion and Vertical Motion Sensors” and also International Patent Publication No. 2012/037292 A1 to Brune entitled “Method to Improve Spatial Sampling of Vertical Motion of Seismic Wavefields on the Free Surface of the Earth by Utilizing Horizontal Rotational Motion and Vertical Motion Sensors”.